Effect of Cr Content on the Microstructure of Casting Infiltration Layers: Simulations and Experiments

Abstract

High chromium cast irons are commonly used as casting infiltration layers in the applications of wear resistance. The formation mechanism of the casting infiltration layer is essential to better develop the surface wear resistance materials using the casting infiltration method. In the present work, casting infiltration layers with various Cr contents were fabricated in situ on the surface of parent ZG45 steel. CALPHAD-type calculations using Thermo-Calc software, SEM, EDS and microhardness tests were performed to study the effect of Cr on the microstructure and hardness of casting infiltration layers. All the microstructures of casting infiltration layers were composed of pearlite matrix and eutectic M₇C₃ carbide. With the increase in Cr content from 7.01 wt.% to 17.20 wt.%, the amount of M₇C₃ carbide increased from 5.05 vol.% to 13.12 vol.%, resulting in the increment of microhardness. With the aid of simulations, the solidification behavior and formation mechanism of casting infiltration layers were revealed.

1. Introduction

In fields such as aerospace, mining, metallurgy, rail transit and weapons, many failures of the related equipment are caused by abrasion wear of the key components [1,2,3]. In order to improve the wear resistance of the materials, alloying elements with high contents are generally needed [4,5]. In most cases, the wear failure of the workpiece occurs on the surface or some specific parts, and the cost of overall alloying with expensive Cr, Ni, Ti and other elements is high. Surface strengthening technology shows superiority in solving the above contradiction.

Wear-resistance surface layers have been developed through various strategies, such as thermal spraying [6], laser cladding [7] and plasma arc welding [8]. The thickness of the layers is usually less than 2 mm, which cannot well satisfy the demand of many wear conditions. The casting infiltration method, a one-step additive manufacturing technique, is a promising way to produce surface wear-resistance composite material [9]. A casting infiltration layer (CIL) is in-situ realized at the surface of metal material through the infiltration of the molten metal into powder preforms bonded to the wall of a casting mold. Compared with other technologies, surface casting infiltration has the advantages of thick surface layer thickness (usually more than 5 mm), low cost and short production cycle.

Previous researchers have investigated casting infiltration technology extensively [9,10,11,12]. High chromium CILs are mostly researched, due to the existence of high hardness Cr₇C₃ carbide [13,14]. Recently, other high hardness carbide ceramics (WC, TiC, VC, etc.) [15,16,17,18], boride ceramics (FeB, Fe₂B, etc.) [19,20] or oxide ceramics (Al₂O₃, ZrO₂, etc.) [21,22] have been introduced to improve the wear-resistance performance of the CILs. Li et al. [15] fabricated WC particle-reinforced iron matrix surface composites on gray cast iron using the casting infiltration method, and the wear resistance of the composites was 1.5–5.2 times higher than that of the matrix. Xu et al. [17] prepared in-situ micro-nano dual-scale vanadium carbide ceramic-strengthened CILs. The best abrasive and impact wear resistance performances were 2.5 and 1.9 times that of Cr20. The casting infiltration method was used by Fischer et al. [19] to generate an FeB-enriched white cast iron surface layer with a high microhardness of 505 HV on a gray cast iron casting. ZTA ceramic particle-reinforced surface composites with Ti binder modification were investigated systematically by Zheng et al. [22], and the abrasion resistance was improved significantly by up to 5% compared with the Cr26 matrix.

Casting infiltration is a metallurgical process that involves complex thermodynamic and kinetic phenomena. Most of the investigations have focused on the development of a new casting infiltration agent and related technical parameters of the casting infiltration process. The characteristics of higher wear-resistance performance CILs are usually experimentally investigated, which are time, energy and financial consumption. The formation process of CIL has not been well illustrated in previous research. With the development of computational materials science [23,24,25], the CALPHAD (CALculation of PHase Diagram) approach has been proven to be a powerful tool in understanding microstructure evolution and design of materials [26,27,28]. This method can be also used to better realize the formation of CIL.

In the present work, CILs with different Cr contents were chosen as targets to be fabricated on the surface of ZG45, using the casting infiltration method. The purposes of this work are as follows: (i) to investigate the microstructure evolution of as-cast CILs with the approach of the CALPHAD method, and validate the reliability by experimental verification; (ii) to reveal the formation mechanism of the casting infiltration layer and to analyze the formation process of the surface composite materials; and (iii) to evaluate the influence of Cr on the hardness of the surface composite materials.

2. Materials and Methods

2.1. Materials

Medium carbon steel ZG45 with certain strength and toughness was chosen as the base material in the present work. The sand mold casting method was utilized to fabricate CILs with various Cr content on the surface of ZG45. High carbon ferrochrome powders (Cr: 70 wt.%, C: 6 wt.%) and high carbon iron powders (C: 8 wt.%) with a powder size of 60 mesh were utilized as raw materials. Four groups of alloy powders with different Cr contents were designed, as shown in

Table 1. Alloy compositions of casting infiltration agents and casting infiltration layers used in the present work (wt.%).

Designation	Casting Infiltration Agent			Casting Infiltration Layer
	C	Cr	Fe	C	Cr	Fe
CIL-1	6	24	Bal.	2.01	7.01	Bal.
CIL-2	6	36	Bal.	2.01	10.36	Bal.
CIL-3	6	48	Bal.	1.97	13.52	Bal.
CIL-4	6	60	Bal.	1.69	17.20	Bal.

. Adhesive polyvinyl butyral resin and alcohol with a mass ratio of 1:20 were added to the powders, and the mixture was ball milled for 4 h. The obtained casting infiltration agent with a thickness of 5 mm was coated on the surface of a sand mold, as shown in Figure 1, and then dried through ignition. ZG45 steel was melted in a medium-frequency furnace and poured into the sand mold with a casting temperature of 1680 °C. The molten steel infiltrates the pores of the powders, and melt them. A metallurgical reaction simultaneously occurred between the molten steel and the powders, resulting in the formation of the CILs.

2.2. Thermodynamic Calculations

Thermodynamic calculations based on the CALPHAD approach were performed using Thermo-Calc software and the thermodynamic database established in our previous work [27]. The solidification behaviors of CILs were realized through the modified Scheil-Gulliver simulations [29], which assume that carbon can be expected to redistribute quickly in the solid phases during solidification. Back diffusion of carbon was also considered, and graphite was suspended during simulation, due to the difficult nucleation. Vertical sections along the compositions from ZG 45 to different CILs with the variation in C content were calculated to help understand the interface microstructure. The diffusion coefficients of Cr in ZG45 were calculated using the DICTRA module in Thermo-Calc software.

2.3. Microstructure Observation

The chemical compositions of the surface casting infiltration layers were determined using an X-ray fluorescence spectrometer (EDX-1050, Shenzhen, China). The cross sections of the as-cast samples with a size of 5 mm × 5 mm × 15 mm were ground, polished and etched in a 4 vol.% solution of nitric acid and ethyl alcohol. The microstructures were observed using a scanning electron microscope (SEM, Jeol JSM-IT 800SHL, Tokyo, Japan). SEM micrographs were studied by Image J software. The area fraction of the M₇C₃ phase was determined by using the average value from ten different images for each CIL. The elemental distributions at the interface between CIL and ZG45 parent material were analyzed using energy dispersive X-ray spectroscopy (EDS, Oxford Ultim Max 40, Oxford, England).

2.4. Hardness Measurement

The microhardness distribution from the surface to the interior was tested along the surface’s normal direction using an HV-1000 microhardness tester, with a load of 200 g and a dwell time of 10 s. The average value was taken from the determination five times for each sample.

3. Results and Discussions

3.1. Microstructure of as-Cast CILs

The actual chemical compositions of Fe, Cr and C in the obtained CILs were listed in Table 1. CILs with different Cr contents were generated on the surface of ZG45. The concentrations of Cr and C in CILs were diluted to about 1/3 of those in the casting infiltration agent. With the penetration of molten steel during the casting process, the casting infiltration agent was melted, resulting in the separate solidification of CILs and the matrix. Thermo-Calc software was utilized to simulate the solidification behaviors of the CILs, including the solidification paths and mole fractions of the M₇C₃ (M = Cr, Fe) phase. The modified Scheil–Gulliver model was applied. Calculated solidification paths for the CILs with various Cr contents are presented in Figure 2. The diffusion paths of all CILs are the same and are as follows: liquid → liquid + γFe → liquid + γFe + M₇C₃. According to the calculated results, primary γFe precipitates from the liquid phase at about 1380 °C. When the temperature drops to 1231~1286 °C, M₇C₃ and γFe phases precipitate from the residual liquid phase at the same time, and eutectic fcc + M₇C₃ forms. With the increase in Cr content, the eutectic reaction temperature increases from 1231 to 1286 °C, and the amount of M₇C₃ carbides also increases. In order to calculate volume fractions from the molar fractions given by the calculations, molar volumes of the various phases are utilized.

Table 2. Comparison of experimentally determined and calculated volume fractions of M₇C_3.

Type	Volume Fractions of M7C3 (vol.%)
	CIL-1	CIL-2	CIL-3	CIL-4
Experimental values	5.05	7.31	10.16	13.12
Calculated values	5.32	8.84	11.84	13.27

lists the calculated volume fractions of M₇C₃ in CILs with different Cr contents, which are 5.32 vol.%, 8.84 vol.%, 11.84 vol.% and 13.27 vol.%, respectively.

Figure 3 shows the SEM images of the as-cast microstructures of the CILs obtained in the present work. It can be observed from the figures that all the CILs are composed of pearlite matrix and eutectic carbide M₇C₃. Figure 4 presents the magnified SEM images of CIL-2 and CIL-4, as well as the corresponding EDS analysis of eutectic carbides. The pearlite matrix and eutectic carbide M₇C₃ can be further confirmed. Primary austenite transformed to pearlite during the cooling process. As can be observed from Figure 3, the amount of carbide content also increases with the increase in Cr content, since Cr is a carbide-forming element. ImageJ software was applied to count the proportion of carbides in CILs. The obtained area fraction can be approximated as the volume fraction of carbide. Experimental values are also listed in Table 2, and compared with the calculated results. The calculated values are in good agreement with the experimental results, indicating the reliability of the CALPHAD method.

3.2. Interface Microstructure of the Casting Infiltration

A macrograph of the metallographic as-cast sample CIL-4 after etching and corresponding SEM image at the interface are presented in Figure 5. As can be observed from Figure 5a, a thick CIL with a thickness of about 7 mm was formed on the surface of the parent material ZG45. Figure 5b indicates that a continuous transition layer with a thickness of about 80 μm was formed between the CIL and ZG45. There were no obvious porosity, crack or other visible casting defects at the interface, suggesting an excellent interfacial bond. EDS line scanning analysis results are shown in Figure 5c. The contents of Cr and C gradually decrease from the CIL to the parent material, indicating the diffusion of Cr and C from CIL to ZG45. The diffusion of Fe is in the opposite direction, and the diffusion mainly exists in the transition layer. A drastic change in the elemental concentrations occurs at the eutectic structure, which indicates that the contents of Cr and C are much higher than those in the matrix of the CIL. The elemental concentrations in the matrix of the CIL are almost the same, and the concentration variations in Fe, Cr and C mainly exist in the transition layer. As shown in Figure 5b, there are no obvious differences between the microstructure at the surface and the inner part of the CILs. The CILs and the parent ZG45 solidified individually. During the solidification process, the elemental diffusion occurred and the transition layer formed. Figure 5d presents an enlarged view of the transition layer near ZG45. The microstructure of the transition layer was only composed of pearlite without other carbides.

Vertical sections along the compositions from ZG 45 to the CILs with the variation in C content were calculated and are shown in Figure 6. The dashed lines on the left of the figures represent ZG45, and the dashed lines on the right represent CILs with various Cr contents. No other phase forms at the interface. The interface microstructure at high temperatures is γFe, which will transform to pearlite. The calculated results are in accordance with the experimental results. It can be also observed from Figure 6 that the melting points of all CILs are lower than the solidus of ZG 45. Even though the CILs and ZG45 parent meta solidify separately, the CILs begin to solidify when ZG45 has already been in the solid state.

The elemental distributions of Fe, Cr and C at the interface of CIL-4 were studied using EDS analysis. Figure 7 presents the SEM images of the interface, as well as the corresponding mapping scanning results of the elements Fe, Cr and C. The contents of Cr and C gradually decrease from CIL to the parent material, while the content of Fe gradually increases. Similar to the EDS line scanning results, the concentration variations in all elements mainly exist in the transition layer, indicating that diffusion mainly occurs in the transition layer. The distributions of C and Cr in the CIL are concentrated in eutectic carbides, where the contents of C and Cr are much higher than those in the matrix.

The impurity diffusion coefficient of Cr in ZG45 steel with the variation in temperature was calculated and presented in Figure 8. It can be observed from the figure that the impurity diffusion coefficient of Cr in the liquid state is about 4 to 5 orders of magnitude higher than that in the solid state. It, thus, can be considered that the diffusion of Cr mainly occurs in the liquid state, and the diffusion in the solid phase can be ignored. That is, the diffusion and transition layer formation mainly occurred above the eutectic temperature.

According to the above results and discussions, the schematic diagram of the microstructure evolution of the surface composite material using the casting infiltration method was drawn and is illustrated in Figure 9. The casting infiltration agent was coated on the surface of the sand mold. The alloy powders in the casting infiltration coating formed a skeleton, forming a large number of pores inside. Molten ZG45 molten metal intruded into the pores between the alloy particles, resulting in the increment of the CIL thickness and the dilution of alloying element concentration. The heat of molten metal and the latent heat of crystallization melted the alloy powders. The solidification processes of the CILs and the parent ZG45 were performed individually, and simultaneously, the transition layer was formed due to the elemental diffusion. CILs began to solidify when the ZG45 parent metal had already been in the solid state. Primary γFe precipitated from the liquid phase of the CILs and then eutectic M₇C₃ carbides formed from the residual liquid phase. With the cooling of the castings, γFe in CILs converted into pearlite, while that in ZG45 transformed to αFe and pearlite. A transition layer composed of pearlite was also generated.

3.3. Hardness

The microhardness, including the CILs and the parent metal of the surface composite materials, was measured and presented in Figure 10. The interface between the transition layer and the CIL was set as the original location. The x-axis value was the distance between the measurement point and the interface. The average microhardness of the CILs shows obvious fluctuations, due to the existence of eutectic carbides and decreases sharply in the parent ZG45. The transition layer eases the composition and performance differences between the CILs and the parent material. The average microhardness of the CILs increases from 402 HV to 453 HV with the increment of Cr content, resulting from the higher volume fraction of the M₇C₃ carbides formed.

4. Conclusions

CALPHAD-type calculations and experimental results were combined in the present work to investigate the effect of Cr on the microstructure and hardness of casting infiltration layers. Four groups of CILs were prepared using the casting infiltration method on the surface of ZG45. The solidification paths of the CILs and several related vertical sections were calculated to study the solidification behavior, the amounts of carbides and the formation of the transition layer. In this study, the following conclusions can be drawn:

(1) The modified Scheil–Gulliver simulations on the solidification behaviors of the CILs predicate that all the diffusion paths are the same and are as follows: liquid → liquid + γFe → liquid + γFe + M₇C₃. SEM micrographs indicate a dispersion of eutectic M₇C₃ carbides in pearlite. The transformation of austenite to pearlite occurs in the following cooling process.

(2) With the increase in Cr content from 7.01 wt.% to 17.20 wt.%, the amount of eutectic M₇C₃ carbide increased from 5.05 vol.% to 13.12 vol.%, according to the SEM images using ImageJ software. The thermodynamic calculation gives similar results, indicating the accuracy of the CALPHAD approach.

(3) Thick CILs, with a thickness of about 7 mm, were formed on the surface of the parent material. A continuous transition layer with a thickness of about 80 μm was also formed between the CIL and ZG45. The transition layer was only composed of pearlite without other new phases. These can be also explained by pseudo-binary phase diagrams along the compositions from ZG45 to CIL, with the variation in C content.

(4) The variations in elemental contents mainly exist in the transition layer, indicating that diffusion mainly occurs in the transition layer. The impurity diffusion coefficient of Cr in the liquid state of ZG45 is about 4 to 5 orders of magnitude higher than that in the solid state. The diffusion and transition layer formation mainly occurred above the eutectic temperature.

(5) The formation process of the CILs can be summarized as follows: penetration of molten steel into casting infiltration agent and molten of the agent; solidification of the parent ZG45 and diffusion of the elements through the interface; solidification of the CIL and formation of the transition layer; solid-state phase transformation in CIL, transition layer and the parent materials.

(6) As the Cr content in the CILs increases, the average microhardness increases from 402 HV to 453 HV, resulting from the higher volume fraction of the M₇C₃ carbides formed.